1. Technical Field
The present invention relates generally to an improvement on a high-pressure pump for use in a common-rail injection system for diesel engines for supplying high-pressure fuel to the engine.
2. Background of Related Art
A common-rail injection system is known as one of fuel injection systems for diesel engines. Japanese Patent First Publication No. 64-73166 teaches a conventional common-rail injection system. This common-rail injection system has an accumulator pipe referred to as a common rail connected to all cylinders of the engine and supplies through a high-pressure pump a desired quantity of fuel to the common rail to maintain the fuel pressure therewith in constant. The fuel stored within the accumulator pipe is sprayed into each cylinder through an injector with given timing.
FIG. 1 shows, as one example, a conventional high-pressure pump for use in the common-rail injection system. The high-pressure pump includes a plunger 92 which is moved vertically by a cam (not shown) within a cylinder 91 and defines a pressure chamber 93 between an upper wall thereof and an inner wall of the cylinder 91. Disposed above the pressure chamber 93 is a solenoid valve 94 having a valve head 96 for establishing and blocking fluid communication between the pressure chamber 93 and a low-pressure path 95.
When a coil 97 of the solenoid valve 94 is deenergized, the valve head 96 is brought into an open position so that the fuel is allowed to be fed by a low-pressure pump (not shown) into the pressure chamber 93 through the low-pressure path 95 and the clearance around the valve head 96 during downward movement of the plunger 92. Alternatively, when the coil 97 is energized, the valve head 96 is attracted upward into engagement with a conical valve seat 98 to block the fluid communication between the pressure chamber 93 and the low-pressure path 95. The upward movement of the plunger 93 causes the pressure of fluid in the pressure chamber 93 to rise so that the fluid is discharged to the accumulator pipe from an outlet path 99 opening into an inner wall of the pressure chamber 93.
During the upward movement of the plunger 92, the increased fuel pressure within the pressure chamber 93 acts on the valve head 96 to urge it into a closed position. This causes the valve head 96 to be held closed once it is seated on the valve seat 98 even when the coil 97 is energized. In order to avoid this problem, the conventional high-pressure pump controls the valve-closing timing under the so-called prestroke control to adjust the flow rate of the fuel supplied to the accumulator pipe. Specifically, the supply of a desired amount of fuel to the accumulator pipe is accomplished by holding the valve head 96 opened to discharge part of the fuel sucked into the pressure chamber 93 to the low-pressure path 95 until the amount of fuel within the pressure chamber 93 reaches a desired value, without closing the valve head 96 immediately after the plunger 92 starts to move upward, after which the valve head 96 is closed.
However, an increase in discharge rate of the high-pressure pump due to a rise in speed of the engine gives rise to the problem that the valve head 96 is closed by itself even if the solenoid valve 94 is not energized. This is because the fuel pressure within the pressure chamber 93 acts directly on the bottom of the valve head 96, and the fuel pressure produced by part of the fuel flowing through an orifice defined by the valve head 96 and the valve seat 98 to the low-pressure path 95 urges the valve head 96 into the closed position during the upward movement of the plunger 92. This may result in a failure in flow rate adjustment.
The above problem may be alleviated by lengthening the stroke of the valve head 96 or increasing a spring pressure of a return spring for the valve head 96. However, in either case, the valve-closing response is lowered. The lowering of the valve-closing response may be avoided by increasing the electric power applied to the coil 97 or the size of the solenoid valve 94 to increase the magnetic attraction produced by the coil 97, but results in increases in cost of electric power and production of the solenoid valve 94.
The above high-pressure pump also has the following drawback. A time lag always occurs between input of a valve-closing signal to the solenoid valve 94 and a time when the valve head 96 is seated on the valve seat 98 to block the fluid communication between the pressure chamber 93 and the low-pressure path 95. The valve-closing timing, thus, needs to be controlled taking this time lag into account. However, when the engine speed rises to require an increase in discharge rate of the high-pressure pump, it will cause the timing with which the valve head 96 is opened and closed to be late.